How can I render with GPU on iMac 09' NVIDIA GeForce 9400 256 MB?
It's difficult to find clear information, but if Wikipedia is to be believed, the GeForce 9400 is supported by CUDA 1.x, but not 2.x which is what Cycles uses (look for the chart). This would be why it says "found 1.1," which is what comes with the driver.

That being said, there seem to be rumors in the deep web... whispers of dark magics... that will allow the 9400GT to run some newer versions of CUDA. Of such things... I hope never to be openly spoken in the halls of men.